Raised Beds - soil volume

Morning everyone...in the next week or so I'll be having 3 raised flower beds made for the top of the garden and I wonder if anyone can help with the amount of soil / compost I will need?
The dimensions of the beds will be 1800mm x 600mm x 600mm and I have done some calculations from what I have read on the internet and come up with 648L of soil / compost per bed.
Here is the calculation
1.8 x 0.6 x 0.6 = 0.648 cubic M = 648L
I have read that 1 cubic meter = 1000L
So can anyone confirm the above calculation and if I have done it wrong then possibly help out with the rght solution.
Thanks very much
Andy...
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Looks about right, the '1 tonne' bags of topsoil are usually around 700-800 litres, so it should be spot on!
Thanks for the help guys...I had never thought of putting rubble in the bottom which will help reduce the amount of soil / compost I need, cheers Tetley.
Good call on the bulk bags, Bob...I might be able to get away with a couple of 800 litre bags, so would I be OK filling the beds with half cheap top soil and then the other half with something like John Innes No.3?
Useful for me too Sheps as we're having a big raised bed out front. Very good idea about the rubble, would never have thought of it otherwise.